1. Field of the Invention
The invention relates to purge gas systems in a lithographic projection apparatus including:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging an irradiated portion of said mask onto a target portion of said substrate.
2. Description of the Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) of a substrate (silicon wafer) which has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions which are successively irradiated via the mask, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from International Patent Application WO97/33205, which is incorporated herein by reference.
In general, lithographic apparatus contain a single mask table and a single substrate table. However, machines are becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO98/28665 and WO98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is at the exposure position underneath the projection system for exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge a previously exposed substrate, pick up a new substrate, perform some initial measurements on the new substrate and then stand ready to transfer the new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed; the cycle then repeats. In this manner it is possible to increase substantially the machine throughput, which in turn improves the cost of ownership of the machine. It should be understood that the same principle could be used with just one substrate table which is moved between exposure and measurement positions.
To reduce the size of features that can be imaged, it is desirable to reduce the wavelength of the illumination radiation. Wavelengths of less than 180 nm are therefore currently being contemplated, for example 157 nm or 126 nm. However, such wavelengths are strongly absorbed by normal atmospheric air, leading to unacceptable loss of intensity as the beam traverses the apparatus. Furthermore, contaminantsxe2x80x94which may be introduced by, for example, outgassing of the photoresist layer on the substratexe2x80x94may adsorb onto certain optical elements, such as that lens element (of the projection system) that is nearest to the substrate. The undesirable adsorption of such contaminants will, in general, also lead to detrimental intensity loss. In order to solve these problems, it has been proposed to flush the apparatus with a flow of gas, the gas being substantially transparent to the illumination wavelength, e.g. nitrogen (N2). However, nitrogen gas of the purity necessary to avoid absorption of the exposure radiation, and in the quantities necessary for a flush of the whole apparatus, is expensive.
It is an object of the invention to provide a lithographic projection apparatus, especially one using radiation substantially absorbed by atmospheric air, in which the consumption of purge gas is reduced.
According to the invention there is provided a lithographic projection apparatus including:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate;
a projection system for imaging an irradiated portion of said mask onto a target portion of said substrate; characterized by:
a compartment closely surrounding at least one of one of said first and second object tables but not surrounding either said illumination system or said projection system, said compartment, in use, being supplied with a purge gas more transparent than air to the radiation of said projection beam.
According to the invention there is also provided a lithographic projection apparatus including:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging an irradiated portion of said mask onto a target portion of said substrate; characterized by:
a purge compartment provided between said projection system and said second object table and fixed relative to said projection system.
By providing a compartment closely surrounding either one of said object tables or in the space between the projection system and the substrate table, the volume that must be purged can be substantially reduced, as compared to purging the whole apparatus. As well as the direct saving in purge gas consumption as a result of the reduction in the volume being purged, there are further reductions, since contamination of the purge gas can be reduced, allowing additional reuse, as can leakage of the purge gas. Additionally, the time taken to purge the system back to a sufficiently clean state of operation after the apparatus has been shut down or opened, e.g. for maintenance, is reduced.
Particular additional advantages can be achieved in step-and-scan apparatus where the compartment can be arranged to surround and move with the object rather than surrounding all of the substrate or mask table, drive arrangements and associated components such as sensors. This can be achieved using a combination of a frame, formed as part of the object table, moving between fixed parallel plates, or by forming the object table into a box substantially surrounding the object. Where the compartment is to be formed between the projection system and the substrate (wafer) these items can themselves form opposite sides of the compartment, which may then be defined by ducts fixed relative to the projection lens and forming a frame around the space traversed by the projection beam. A preferential embodiment employs gas flow velocities which are sufficient to completely or partially prevent contaminants (e.g. as introduced by resist outgassing) from adsorbing onto optical elements in the apparatus. Such velocities may, for example, be or the order of about 1 m/s.
The invention also provides a method of manufacturing a device using a lithographic projection apparatus including:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging irradiated portions of said mask onto target portions of said substrate; the method comprising the steps of:
providing a mask bearing a pattern to said first object table;
providing a substrate provided with a radiation-sensitive layer to said second object table;
irradiating portions of the mask and imaging said irradiated portions of the mask onto said target portions of said substrate; characterized by the step of:
providing purge gas to a compartment closely surrounding at least one of said first and second object tables but not surrounding either said illumination system or said projection system, said purge gas being more transparent than air to the radiation of said projection beam.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices (dies) will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-0672504.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9cexposure areaxe2x80x9d or xe2x80x9ctarget portionxe2x80x9d, respectively.
The radiation used as the projection beam should not be seen as being restricted to the cited examples of radiation having a wavelength of 157 nm or 126 nm; it is conceivable that other wavelengths or types may also be used in the present invention.